One item of evaluation of receiver performance of a mobile terminal includes out-of-band blocking performance in which receiver performance of a downlink signal is degraded due to an influence of out-of-band interference signals.
In measurement of out-of-band blocking performance, basically, an interference signal of an unmodulated continuous wave (CW) at a predetermined level is combined with a downlink signal to be applied to a mobile terminal, a resultant signal is transmitted to the mobile terminal. An operation of measuring a throughput of the mobile terminal for the downlink signal is performed while changing the level according to a rule while changing the frequency of the interference signal at a predetermined step (for example, 1 MHz step) from an initial value (for example, 1 MHz) to a last value (for example, 12750 MHz) other than a frequency band of the downlink signal (hereinafter referred to as a downlink frequency band), the mobile terminal is regarded to be passed if the throughput is equal to or greater than a defined value at all interference signal frequencies other than the downlink frequency band, and the mobile terminal is regarded to be failed if the throughput does not exceed the defined value at any frequency.
Here, a broadband signal generator that sequentially generates a very broadband range from 1 MHz to 12750 MHz as described above with a determined frequency step and signal level is required as means for generating an interference signal of a CW.
However, an output of such a broadband signal generator includes a spurious component such as harmonics or intermodulation waves due to nonlinearity of an internal amplifying element or the like, in addition to a fundamental wave component that is used as an interference signal.
A level of this spurious component is generally lower than that of the fundamental wave component. However, if the spurious component falls into the downlink frequency band, it is expected that the spurious component influences receiver performance of the mobile terminal.
In order to solve this problem, in the related art, a filter that suppresses a signal component in a downlink frequency band from an output of the signal generator is used, and a signal in which the signal component in the downlink frequency band is suppressed by this filter is regarded as an interference signal, combined with the downlink signal, and applied to the mobile terminal that is a test target.
However, in recent years, a carrier aggregation (hereinafter referred to as “CA”) scheme capable of high-speed information transfer by simultaneously using a plurality of downlink signals having different frequency bands has been adopted. When the out-of-band blocking performance is measured for a mobile terminal corresponding to this CA scheme, it is necessary to prepare filters respectively corresponding to the plurality of downlink frequency bands.
Since an arbitrary band allocated in a range from an approximately 800 MHz band to a 3.5 GHz band is likely to be used at present as the downlink frequency band that is used in the CA scheme, frequency variable filters capable of arbitrarily changing a signal suppression band in a range of 800 MHz to 3.5 GHz should be prepared by the number of downlink frequency bands in a case where there are a plurality of downlink frequency bands as described above. There is a problem in that a configuration is complicated and a cost increases.
Further, it is also conceivable that a process of measuring a throughput for each frequency of the interference signal for one of a plurality of downlink frequency bands in a state in which the frequency of the frequency variable filter is fitted to the downlink frequency band with only one frequency variable filter described above, fitting the frequency of the filter to the next downlink frequency band, and then measuring the throughput, is repeated.
A processing procedure thereof will be described with reference to a flowchart of FIG. 5.
First, as a preparation step, a link establishment process is performed so that communication with the mobile terminal can be performed, and transmission power of the test device and the mobile terminal is set (S1).
Then, a variable i for designating an interference signal frequency is initially set to 1, the interference signal frequency Fif(i) is set, a variable j for designating one of a plurality of downlink signals is initially set to 1, and a signal suppression band of the filter is fitted to a frequency band of a j-th downlink signal DLj (S2 to S5).
In this state, a throughput for the j-th downlink signal DLj is measured. When the measurement ends, the variable j for designating the downlink signal is incremented by 1, the processes S5 and S6 are repeated, and throughputs for the all downlink signals are measured (S7 and S8).
Next, it is determined whether or not all of the throughputs of the downlink signals DL1 to DLN sequentially obtained through this measurement are equal to or greater than a defined value R determined by a standard. If all of the throughputs are equal to or greater than the defined value R, a pass determination in which out-of-band blocking performance at this interference signal frequency satisfies a criterion is performed. In a case where any of the measured throughputs does not satisfy the defined value R, a fail determination in which the out-of-band blocking performance at the interference signal frequency does not satisfy the criterion is performed (S9 to S11).
After the pass and fail determination for one interference signal frequency is obtained in this manner, a process of switching the interference signal frequency to the next frequency and executing the processes S3 to S11 is repeated, a determination result or the like is displayed in a step in which the pass and fail determination for the last interference signal frequency is obtained, and the measurement ends (S12 to S14).
A method of measuring a throughput of one of the plurality of downlink frequency bands in a state in which a frequency of a filter is fitted to the downlink signal, using one filter as described above, and then, repeating a process of fitting the frequency of the filter to the next downlink frequency band and measuring the throughput is described in, for example, Non-Patent Document 1.